Chemical processes and composition



Patented Nov. 13,1951

UNITED A STAT-ES ff'I- ENT O'F F-l-C E No Drawing.

This invention relates to 'processes for building up thesize of particles in aqueous'silica' sols and to stable sols therebypro'duced, and is more particiilarly directed't'o such-processes in which a heel is formed by heatingto-a temperatureabove 60 C. an aqueous sol of 'silicaparticles ofjless than 10 millimicronsidiamete'r, asilicafls'oll'containing'particles of less'than 10 millimicronsdiameter is added to said heel, and the addition and heating is continued 'un'tn at least five times as much silica hasb'een added to the heel as was originally present, and is further directed to silica sols, stable against gelation, containing such built-up particlesv and'characte'ri'zed by having a silica :alk'ali ratio of from 60:1 to 130:1, 'by containing .discretesilic'a particles having a molecular Weight, as determined by light-scattering, of more than one-half million, by having a relative Viscosity, atlO. per cent'SiOz, of from 1.15 to 1.55, and, in a preferred embodiment, by containing from 20 to 35 percent by weight of SiO2.

. Historic methods for. producing colloidal silica solutions gave products which had :mue commercial value because of their tendency to'g'el upon standing. Varidus methodshave been'pr'oposed for improving the stability of colloidal silica solutions, such a by dialysing out electrolytes which were thought to contribute instability, but all ofthese procedures have been slow, tedious,

and not adaptable to commercial production.v

Hence, although a need iforcolloidal silica solutions has long been recognized, it is onlyirecently that solutions having practical stability have become commercially available.

In United States Patent "2,244,325 issued June- 3, 1941, to Paul G.'Bird,there is described a proc-' ess for making colloidal silica solutionsby passing alkali metal silicate solutions through ion exchange resins. "The method gives a product which is relatively stable evenjafter concentrating to a silica content of from to 15 per cent, and is well adapted to large scale commercial operations. Although the e'filuent is obtained from' the ionexchanger with an-SiOZ content of about 3 to 3.5 per cent, the patent states that this.

. techniques ofthis invention woul'dibe more'istable' against "gelling at ,high concentrations, "and any ApplicationDecember .15, 1948, Serial No. 65,536

reasonof itsihigh Water content and it cannot be usedwhere the'dilution caused-bythis water content is objectionable. For instance, when the colloidal silica' is to' be "incorporatedinto elastoinei's. such as: rubber .the watercontent-of "a 15 per. centfsoluti'on of si'licaconstitutes a major problem. ,In'this and other uses where the fin ished productiis 'in' substantially 'dry'form' the advantages ofhavinga solution of'hi'gher, solids content'than" .15 per cent is readily apparent;

Furthermore; for the amount oflactive ingredient been found that particles-in aqueous" silica sols can be built'up to any desired sizeand'stable sols may thereby be produced by processes-comprising forming a heel, byheating to a temperatureabove ?"C. an aqueous Jsol "of' silica parti'cles "of'less" than '10 millimicrons diameter, adding "to i said heel asilica *sol containing particles of lessl'than 1O millimi'cronsdiameter; and continuingthdad dition and heating until 'at least5" times as 'much silica has been'addedfto thei'heel' as was originally present. By suchbuild-eup' processes solsmay be produced which are amenable."toiconceiitration even to as. high as 35 per ."centSiQzI without gelline; "the sols being characterized by. having" a silica:alkaliiratio; of from 60 I "to 13,0 1,1 by containin idiscr'ete ';silica parti'cles; having a molecular. weight, asfdetermine'd by; light=scattering; ,of

more: than one lialf" million, by having a "relative viscosity; at 10; per cent iSiQz, of fromLIS $0 1.55; andji'n the concentratedform';bycontaining from' zo'wss per cent by weight biSiQz. It is not readily. apparent from 1 theoretical considerations that 'the silica products ,of11the explanation -which'mightibeofiered would not be easily susceptible-to experimental proof. ex-

fNow according t'othe 'present "invention it"has' planation which appears to fit the facts is that colloidal silica, such as is prepared from sodium silicate by ion exchange, originally contains particles of relatively low molecular weight and which have a large volume in relation to the Si02 units contained therein. Upon heating, these particles undergo polymerization and densification. By processes heretofore known, such heating results in gelation of the sol, either immediately or after some concentration has occurred by evaporation of water. ,However, if the linity. This is a simple molar equivalence; for

' example, 94 parts by weight of K20 is equivalent silica:a1kali ratio and molecular weight of the starting sol are correctly controlled and ifdensification of voluminous particles is effected in the presence of particles already densified, by adding a second sol containing the voluminous particles to a first sol containing the already-densified particles, the voluminous particles appear to combine with the densified particles and be densifled in combination. The resulting sol thus contains a relatively small number of larger dense particles which have less tendency to unite with each other and hence the sol has less tendency to gel. This theoretical explanation is, of course, not necessary in order to define the invention; sufiice it to say that the processes as operated and the products obtained are characteristically different from those heretofore available.

In describing this invention certain of the terms used are capable of more than a single interpretation, and while the terms are here used in their usual and ordinary meanings, the following definitions are given in order to avoid any possible misunderstanding.

The term silica includes silicon dioxide, SiOz, in any of its various forms. In aqueous colloidal solutions silica is present in chemical association with various amounts of water. Silica may be regarded as including the family of substances theoretically obtainable by dehydration of orthosilicic acid, Si(OH)4, and the products formable by splitting out water between two or more of such substances. Thus, the dehydration products of orthosilicic acid may be regarded as monomers and the products formed by condensing out water between two or more such monomers are designated as polymers of silica or silicic acid. This condensation reaction occurs spontaneously and may proceed to the point where the polymer has an infinite molecular weight and exists as a gel.

In this description, silica sols include colloidal dispersions of silica in aqueous media. Colloidal solutions will be understood to include dispersions in which the size of the silica particles is from 1 to 100 millimicrons. The sols contain particles readily observable by means of an ultramicroscope and may even possess turbidity observable to the unaided eye.

The silicazalkali ratio of from 60:1 to 130:1, which is a limitation, upon sols used in and produced by this invention, refers to the weight ratio of total silica, expressed as $102, to total alkali, expressed as NazO. For instance, if the total silica dispersed in a particular sol is 30 per cent S102 by weight, and the total alkali is 0.5 per cent Nazo by weight, the sol has a ratio of 300:5 or 60:1. Such a sol would be within the operable range of ratios covered by the term "from 60:1 to 130:1.

The alkali, although expressed in the ratio as NazO, may be present as a base of another metal or ammonia. In such a case, the weight of the base actually present is calculated to a weight 01151320 which is equivalent on the basis of alkato 62 parts by weight of NazO. As a practical matter the alkalinity present is easily determined a by a volumetric titration with a standard acid, and the acid used is readily calculated to NazO. Organic bases, such as morpholine, may be expressed as NazO by calculation from such a standard acid titration.

The SiOzzNazO ratio of a colloidal silica solution, as the. term is used herein, means the SiOzINazO weight ratio.

'As the proportion of S102 increases in a sodium silicate solution the characteristics of the solution become more those of a silica sol and less of a sodium silicate solution until at a ratio of about 4:1 and above the S102 has the predominating influence on the sol characteristics. The present invention, of course, is concerned with sols in which the ratio is upwards of 60:1 and in such sols the alkali constituent is present in such small proportions that the sols may properly be referred to as colloidal silica sols. On the other hand the alkali constituent, though small, is important for its stabilizing action in preventing the gelation of the silica.

The absolute turbidity of a silica sol refers to its turbidity as determined by light scattering. It is expressed as reciprocal centimeters (cmr and is determined in a solution containing l per cent by weight of SiOz and is determined by means of a light scattering photometer such as that described by P. P. Debye, Journal of Applied Physics, vol. 1'7, May, 1946, or the B. S. light scattering photometer of the Phoenix Precision Instrument Company as originally described by Speiser and. Brice, Journal of the OpticalSociety of America, vol. 36, page 364 (1946). The latter instrument was used in determining the value of absolute turbidity for the feed solutions used according to this invention.

When reference is made to the molecular weight of a silica sol as determined by light scattering the molecular weight is, of course, that of the silica particles dispersed in the sol. This molecular weight may vary widely by reason of the condensation reaction referred to above. The molecular weight of the various silica particles may not be at all uniform and when mention is made of molecular weight in defining such compositions, the weight average molecular weight is meant.

The weight average molecular weight in silica solutions can be determined by a light scattering method adapted from the method described by Stein and Doty, Journal of the American Chemical Society, 68, 159, (1946). According'to the theory of Stein and Doty, the weight-average-molecular-weight, Mw, is a function of the turbidity, T, and the concentration, C, of a polymeric dispersion of solution, according to the following equations:

(Equation a) R=Gas constant in erg/mole of solute/degree e temne at v ance 'is' known.

assesse- 'I Absolute temperature in I. degrees. 1'- Turbidity (extinetion-icoeflicient for scattering at right-angles: mom

(Equation b) 7 lightsource which is .firsterendered monochromatic (5461-13.) by the mounted Corning filters Band then made parallel by a collimating lens intercept and the slope iiby, zfor.-.example,i:-.the method ofleast squares.

The -method used for calculating molecular weight from light-scattering ,data is believed-11:0

give-molecular weight values whichare absolute in the rangebelowabout five million. However,

- as-the linear dimensionsof a particle become-appreciable-say morethan one tenth of the wave-' length of thelight usedinterferencephenomena necessitate a correction, which is knowni as -the dissymmetry factor. Rather-than use thisinvolved mathematical-correction it is preferred: to. corr'elatethe uncorrected molecular weights w-ith actual particle-size-as -observed d-irectly by means ofthe-electron microscope. For the-h-ighly uni form, spherical, dense particles-herein dealt-with, this correlation is entirely -feasible; accordingly-it will be understood that when --molecular-weight values above five million are referred.to,-the

C in conjunction with a small 1.5 cm. diaphragm :molecular weight values uncorrected for dis- D'.' ';Neutral filters to reduce the intensity of the flight during the calibration are inserted before the diaphragm instead of in the position E indicated on Figure l. -A semitransparent glass plate E mounted after the green filters reflects a small fraction of the beam through ori' o'palescent glass plate F and thence into the photometer K, to be compared visually with the'light-scattered at 90 by the solution. A

square glass cell G containing the polymer solution immersed in a water-filled chamber h to -reducereflection at the interfaces, and the water-filled chamber is surrounded by circulating water for temperature control. The light reflected at 90 by the solution is transmitted through a lens J into the other side of the photometer and compared with the standard beam.

. A cone I absorbs the transmitted light by multiple sill at to the incident beam, or on a similarly 45 placed vitrolite surface whose absolute reflect- '-Readings are taken when matched fields are obtained. "The two neutral filters are removed and the block is then replaced by a solution of the polymer of known 5 concentration and again a reading is taken when the-fields are matched. The properties of light made it possible to correlate the dial readings obtained during the calibration with the solution values and to obtain values for absolute H/tu'rbidity is plotted-against concentration for 7 four or five points on a given solution. MW maybe determined by extrapolation of the best straight line drawn visually through these points and use of the intercept in Equation a. More accurate values areobtainedby calculating the .originallypresent in the-heel.

symmetry are mean and that these values-arerelated to the actual particle sizes by the-following formula:

Logic D=0-.5-7 'L0g10- M+1.0

where D=particle diameter inmillimicrons. M=weight average molecular weight bylight scattering, uncorrected for dissymmetry, .expressed --in millions. 1

The figure obtained for the .molecular Weight by the above-described light scatteringtechnique scribing this invention is used in its usualand ordinary meaning and refers to the ratio of theviscosity of the solution to the viscosity of the solvent. In this particular case the solution .is the silica sol, the solvent is water, and the viscosity is measured at 25 C. on a sol containing 501' 10 per cent S102 by weight as indicated. A sol containing less than 10 per cent SiOz may be concentrated up to 10 percent by vacuum evaporation of waterat room temperature-and a more concentrated sol may be diluted back with water to-5 or 10 per cent for measurement. It will be understood that when reference is made to relay tive viscosity of 5 or 10 per cent S102, this means that the determination is made at 5 or 10 percent SiOz, but the sol, before adjustment, may contain more or less SiOz than 5 or 10 per cent. e The viscosity measurements used in determining relative viscosity may be made according to conventional methods-provided they are capable of adequate precision. Measurements made with a capillary pipette, for instance, under properly controlled conditions of temperature are adequate for the purpose.

In carrying out a process of this invention a silica sol feed of the specified characteristics is added to a heel consisting of another sol also having specified characteristics and the .body'of combined sols is heated until its properties are those'required for stability at high concentrations, at least five parts by weight of silica a s feed having been added for each part of silica :.The art" is already familiar with various ways of making silica sols, and any of these methods which gives a sol of the specified characteristics may. be used for preparing the sols to be modified according to this invention. The method selected should give .a sol not highly contaminated with.

electrolytes, as these contribute to subsequent instability. It will be understood however that the alkali which is responsible for the ratio being in the specified range is not to be considered as a deleterious electrolyte. A suitable solmay be prepared by first removing all of the alkali from an alkaline silicate such as sodium silicate. and then adding back an amount of alkali necessary to adjust the ratio to the proper range, or the same result maybe effected by removing alkali from, say sodium silicate, only sufiiciently to leave a sol of the desired ratio.

Among suitable methods for making silica sols are the following:

. 15y dialysis of sodium silicate.The sodium silicate may be neutralized with an' acid and the sodium salt formed may be dialyzed out through a semi-permeable membrane. 7

From methyl silicate.Brimaux, Compt. rend. 98, 105 and 1434 (1884). Methyl silicate is hydrolyzed with Water to give a silica sol.

. From silicon tetmchloride.Williams, U. S. Patent 1,539,342. Silicon tetrachloride is hydrolyzed with water to give a silica sol.

From silicon suZfide.Fleury, U. S. Patent 6 1,931. Silicon sulfide is hydrolyzed with water to give a silica sol.

By electro-osmosis of sodium silicate solution.- Schwerin, U. S. Patent'1,132,394. A dilute solution of sodium silicate is subjected to electroosmosis, a silica 501 being obtained in the anode compartment.

By the electrolysis sodium silicate with a mercury cathode.Collins, U. S. Patent 1,562,940. By electrolyzing a solution of sodium silicate in a' cell fitted with a lead anode and mercury cathode, there is obtained a silica sol.

Obviously, where in the above methods silicic acid is formed by hydrolysis of a silicon compound, it is necessary to remove the .by-products, such as methanol, hydrochloric acid, or hydrogen sulfide, in order to obtain a pure silica sol.

While a silica sol prepared by one of the foregoing procedures may be employed as the starting point in a process of this invention, particularly if the sol is freshly prepared, the prbcedures are subject to a number of objections, such as being slow and being poorly adapted to commercial production, as discussed above. 7

A process which is subject to none of these objections and which gives a silica sol especially Well adapted for use in a process of this invention is that of the Bird Patent 2,244,325 mentioned above. According to this method an alkali metal silicate solution such as sodium silicate is passed through an ion exchange material which removes most of the metal ions and gives a silica sol of the desired SiOeINazO ratio. Alternatively, all the metal ions may be removed, in which case the sol may then be adjusted to the desired ratio by adding the requisite amount of sodium silicate or sodium hydroxide solution. In making such adjustment the sol should not be permitted to remain long in the pH range of to 7 since in this range it is unstable.

The fresh eiiluent from an ion exchange process of the Bird patent, with the ratio suitably adjusted as just described, is ideally adapted for modification by a process of this invention. The

absolute turbidity is substantially less than 0.0226 cm.- at 1 per cent SiOz with 547 millimicron.

wavelength light, and the relative viscosity, atv 15 per cent S102, is more than 1.29. This so] is therefore suitablefor adding to the second sol.

- Thefirst sol will hereinafter be referred to as feed and the sol to which it is added will be referred to as heel. This terminology is derived from the practical application of the processes in an evaporator, where the second sol is the heel present in the evaporator and feed added thereto. l

. The silica content of the feed may be stated. The feed 'must not, of course,.be .a gel.

The customary processes Of. making silica sols: tend to give gels as the concentration of silica. increases, and accordingly caution should be o bserved to keep the silica content downto where; no gelling occurs. With silica sols prepared by v, ion-exchange a feed containing from 2 to 6 per cent SiOz is entirely feasible, and more particu larly, a feed containing about 2.5 per cent SiOz is especially satisfactory. v g

The size of silica particles in the feed sol and in the sol which is heated to make the: heel J; should be less than 10 millimicrons in diameter.

Particles of this size range may be observed by means of an electron microscope. In determin ing the size of silica particles by the electron microscope there is some densification efiected in the preparation of the sample. This is minimized by drying the sample at roomv temperature, under vacuum. Thus, the particle size limitation of 10 millimicrons referred to in the description of this invention is the size of particle as observed on an electron microscope on a sample originally whereas the relative viscosity of the heel is spec- I ified at 10 per ,cent S102. A sol having a relative,

viscosity more than 1.29 at 5 per cent has a rel ative viscosity of more than 1.60 at 10 percent.

The rate at which the f-eed is converted to a suitable heel is proportionalto the temperature; I that is, the higher the temperature, the faster I the conversion takes place. Superheating, un-

der pressure, may be employed, but for most pra ctical purposes the rate of conversion at or about the boilingtemperatureis sufliciently rapid. It,

is preferred to heat to a temperature above abou 60 C. in any event. a

The heating at this point should not be continued too long, because simultaneously with lowering of viscosity there is an increase in molecular weight, and unduly prolonged heating will cause the molecular weight to exceed the fifty million, as determined by light scattering, which is the upper limit of molecular Weight for the silica in the heel.

When the relative viscosity of the heel has I been reduced to from 1.15 to 1.60 measured at 10 per cent S102, addition of fresh feed sol may be started. The heating is continued and the fresh sol is fed in at such a rate that the relative viscosity of the combined sols is maintained in the range from 1.l5 to 1.55 measured at 10 per cent S102.

the first sol is the as hi ii as it is feasible to make Within the specification The correct rate of feed'for ajgiven set of conditions may be determined readily by 1 drawing-off a sample .of: the combined; sols,- ad-- ausvgooz 9 justing -it to 10- '-per-= cent Si-Gz; and determining the relative viscosity-y or by determining the -rela tire viscosity of the combined" sols-at the sili'ca content presentand caIcuIati-n the-value at '10 p'er-lcent 819aby 'extrapolation from: a--previously determined curve of-siii'ca concentratiorr plotted against-relative viscositys Theaddition of the feed sol is continued until atleast '5 parts by weight ofiSiO-have beenaddd perpart of SiOir originally present in the-heel. The proportion ofsilica' so added'may be in (ex-'- cessof 5' parts for each original part. Thus, when'it is' desired to=concentrate a so1-fr'om 2 'per cent SiO to 30 percent SiOz without loss-"of volume, the-proportion added I is l4 toone.

The combined sols are heated in a process' of this invention, preferably above about 60 0., and it isPespeciaHy preferred-toheat'at or above th'e normalboiling-temperature. The heating-maybe accompanied by evaporation of water, or may be carriedxout under pressure or refiux so that no weterisi st. Suchheatiiig may then be followed b'y concentration of the soliri any desired manner, such? as by atmospheric evaporation, vacuum evaporation or eleotrodcantation orsimilar techniques;- The molecular weightof'the silica particlescincreases: during heating and produces arsokwhichisnotonly'more stable against gelling atttheexisting concentration Of'SiO'z but is amenable to concentration'without gelling;

The .heating ofxthasmall silica particles of-the feed. Lsol in: the. presence of" the larger silica particles insztheheel efi'ectsa build-up of the smaller rparticles upon the larger. This buildupsproduces' afamil'ylof new silica particles which arei'substantial-ly' spherical in: shape, surprisingly dense foreparticles of non-igneous origin, and remarkably uniform in size; When the buildupqorocess: is continued-. long enough the particle size:(as;direct1y 'observedlby electron micrographs) increases intoithes-rangeof l5-to l30imillimicrons average difihlhtlii Even with particles in thlsisize tinueduntil the-concentration of SiOz in the sol iST'8.t.'- any. desired figure; which may: evenabe in excess of 30 pertinent-S102. When it is: consideredi-that the efiluent from" the ion exchanger mayacontain. as little as about 2 percent SiOz it w-ill. be seerr that suchiconcentration represents as manyfoidt' increase in: the amount: of silica present per unit volume "of sol. Such aniincrease in concentration would be impossible without gelling, -were it I not" i for the densification. and growth of theparticlesreiiectedzby the heating, as indicated by;the.decrease;in relative viscosity.

In :another. preferred; embodiment of this in"- vention the heating is carriedout While maintaining the-relative viscosity of the combined sols initherange from; 1.15to:.1;55 at1l0'per cent SiOz byisuitable control ofthe' 'rates of adding feed and ion-heating, as above, but-a portion of the combined sol body--is .continuously withdrawn, after reaching the proper relative viscosity. Water may bwsimultaneously evaporated; The rateslof feeding .fresh sol;'evaporating water, and drawing off productmay be so adjusted as to maintain the volumewf the combined sol body substantially constant Thisp10cess"- gives-a product i ofmuch" highermolecular weight; but

10 which nevertheless characterized byhaving a relativeviscosity, measured at 10- per cent S102, of-'- from -'1'.15 to -1L55 andby -being-stab1e against gelling evenwhen concentrated above about 30 percent SiOi.

It will be understood that the heating of the combined'heel and feed sols may be commenced immediately; and it-is especially advantageousto heat the heel about 1 to :the boiling point before adding thefeed sol and :continue such heating during the addition. This method of operation facilitates holdingthe volume constant;

The heating-may be carried out in any suitable apparatus, the art beingiwell' acquainted 'wi-th the design of apparatusoie-this type and particularly with efiicient designs for rapid evaporation'ofwater. Vacuum evaporators-may-beused, but the-- vacuum should not be so high as to lower the temperature to a point-where the heating time is unduly prolonged.

A- silica sol produced by a process of-this-invention is characterized by having a silica: alkali ratio of-from" i'1 to l30zl'and preferablyby having a ratio offrom"': 1'to 100:1. It contains discrete silica particles having a molecular weight, as determinedby light'scattering, of more than one=ha1f'million. The product has a relative viscosity, as-measuredat10 per cent SiO-i of from: 1.15 to 1'.55.- Ina' particularly usefulaspect of the invention the 1 product contains from' 20 to 35 percent by weight of S102.

The silicasols of -this-invention are-usefulin a variety of purposes. They are unique in th'at evenat 20 to35 per cent by weightSiO'z theyare indefinitely stable-1: against gelling at ordinary temperatures and arestable-iorat least one month: against gellihg at 95"C. B'y reason of their concentration they are well adapted for suchpurposes as: incorporation into elas-' tomers': such: as rubber; but even when diluted they='are highly usefulier such purposes as=improving the slip resistance'of coatingsmade from Wax emulsions containing..them. Such use in wax emulsions is disclosed and claimed in copendihg application Serial 'No. 775,375; filed September 20, 1947, by Ralph Iler.

The invention will be' better understood by reference to the following illustrative examples;

Example 1- A silica sol-havingan siOzzNaz0' weight-ratio of :1 was made'up bypassing a. dilute solution of sodiumsilicatehaving'a 3.25:1 ratio through a bedof" an ion-exchange resin in the hydrogen form. The sol contained-386 per cent $102 by weight, had-an absolute'turbidity of less than .0226- cmr at 1 per cent SiOz with 547 milli'mi cron wavelength light, and a relative viscosity at 5 per cent'siOz of more'than :29.

A heel was made by heating this silica sol cfor 1 /2 hours under reflux. At theend of'this time theheelhad "a relative viscosity in; the range --of from 1."l5 t0-1.60" at 1-0 'per centSiO; and the silica-particles hadamolecular weight of less than fiftymill-ion as determined by light scattering. The $102 :NazO ratio "remained at 80:1.

To 200 'parts-by'yolume of this heel there was added 1000 parts by-volume of'the original sola The addition was-made-at a uniform rate over a peri'od' of '3 /2 hours; and "theloody of combined sols was-maintained at its boiling temperature, evaporated water beingreturned as-=reflux-.

The produot thus obtained "was'-a'--stable silica. sol which" had 1 not gelled after" standing for 5 months-at ordinary 'temperaturess The molecucentration to 20 lar weight of the silica was 3,700,000 as determined by light scattering, and the relative viscosity at 10 per centSiO- was in the range .from 1515i!) 1.55. This product was suitable for conper cent SiO2 or above,without gelling. V t ,In contrast a product made in a similar mane ner'except that the feed was heated under re- ,fiux for 1 /2 hours before adding to the heel, had a molecular weight, by light scattering, of only 820,000. In this case the relative viscosity of the feed, at per cent S102, was less than 1.29. This product was not suitable for concentration to 20 per cent S102 without gelling.

" Example '2,

A silica sol containing 3 per cent by weight of silica and having an siOzzNazOratio of 87:1; an absolute turbidity of less'than 0.0226 cm.- at 1 per cent SiOz with 547 millimicron wavelength light, and a relativeviscosity'at 5 per cent S102 of. mo re than 1.29 was made bypassing a dilute solution of sodium silicate having a 3.25 ratio through a bed of an ion-exchangeresin in the hydrogen form.

A two hundred part by volume portion of this $01 .Was heated to boiling under reflux, and to the heel so produced was added gradually, over a period of 4 hours, 1800 parts by volume of the original sol, the boiling under reflux being continued during this time. This rate of, feed maintained the relative viscosity in the range from 1.15 to 1.55 at per cent SiOz. K -;The sol thus obtaineduwasturbid, indicating the'presence of silica particlesof high molecular weight. The concentration fo silica in the sol was increased to 17 per cent $102 by evaporating off water under vacuum. The concentrated sol so obtained was relatively stable against gelling, being ungelled after several months storage at room temperature. The SiOzINazO ratio was the same as the original sol.

Example 3 contained the desired quantity of water recycled.

from the ion-exchanger wash operation (which will be described later) and the solution was agitated. In this operation the strength of the dilute silicate solution was adjusted to from 2.0 to 2.4 per cent SiOz.

In thenext operation the sodium ion was completely removed from the dilute sodium silicate solution leaving a dilute solution of silicic acid, that; is, a silica sol. This was accomplished by pumping the contents of the dilution tank through one of two ion-exchangers.- Control of the flow was obtained by means of a rotameter and an automatic shut-off meter. The initial. $1181 involved displacement to the sewer of the water in the ion-exchanger by the silicate solution- ;When a volume of silicate solution had passed through the meter equivalent to the contents of, the ion exchanger the flow was automatically shut off. The valves were then manipulatedso that the emuent was directedto one of 12 two dilute silica effluent tanks and the meter re;- set to permit the flow of the remainder of the idlute silicate batch to the ion exchanger. Water was then fed to the ion exchanger through another rotameter and the same meter as previously used. Again the meter was set to permit the volume displacement of the ion exchangers '(Nalcite I-ICR), which operation will next be described.

The initial step in the regeneration operation was to set the valves so that the effluent from the ion exchanger flowed to the sewer. The required amount of water was then metered into the unit. At the same time additional water was intro.- ducedthrough rotary hydraulic jets located just above the resin bed to assist in breaking up. any

inary wash purged the unit of any silicate solu- I tion whch on contact with the acid to be intro duced next would have gelled. Obviously the formation of any such gel in the ion exchanger would be undesirable. For the regeneration, concentrated commercial sulfuric acid was diluted to approximately 10 per cent H2SO4 in an acid dilution tank which already containtd the cal+ culated amount of water to provide a 10 percent H2804 solution. Agitation was obtained by the use of an air sparger. The outlet valves from the ion exchanger were then set to direct the flow. to

l the sewer by way of a neutralizing tank'wher'e the pH of the acidic waste was automatical1yad= justed to neutrality by the controlled additionof caustic soda. Then the diluted sulfuric acid was introduced to the ion exchanger by means of a water eductor which further out the acidity of the acid to approximately 3 per cent H2804. After the contents of the acid dilution tank had been fed to the ion exchanger, the unit was thoroughly flushed out with water. During this last washing step the flow from the ion ex changer was so directed that a suflicient quantity of the efliuent went to the acid dilution tank to prepare the next batch of 10 per cent acid after which the balance was directed to the neutralizing tank and thence to the sewer.

In the meantime the eflluent silicic acid in the effluent tank was adjusted to the desired SiOzzNazO ratio by the addition of sodium silicate solution from the scale tank. It was then pumped into an evaporator which was of the atmospheric, constant volume type. Here the material was concentrated to 30 per cent S102 content and it.

was then pumped to a product tank for'analysis. The evaporator was equipped with a thermojet compressor which utilized some of the waste heat from the evaporation.

The eiiiuent going into the evaporator had an SiO2:NazO weight ratio of :1.. The effluent had a relative viscosity, as measured at 5 per cent SiOz, of more than 1.29. The volume of the evaporator was about one-third of the volume of each dilute silica eflluent; tank and in order to concentrate to 30 per cent SiOz, three dilute sill ca effluent tankfuls were used. The ratio of silica added as feed to that present in the tank as theoriginal heel was therefore 8:1. The boile ing of the original evaporatorful of silica sol gavehaheel which hada relative viscosity, as

measured at 10 per cent- Si02, of from. 1.15..to 1.60,.a-nd the relative viscosity of thefinalproduct, ..as--measured at-10 per cent SiOz, was from 1.15 to 1.55. The eflluent fed to the evaporator was water-clear and had an absolute turbidity of less than 0.0226 cm.-- at 1 per cent SiOz with 547 .millimicron wavelength light. The heel had a molecular weight asdetermined .by light scattering of-less than two million andthe final product had amolecular weight as determined by light scattering of from 1 to 2 million.

Example 4 Asilica sol-'having'an siozzNaeoweightratio of '78:4': 1 was mad'e by passing a dilute solution of so' dium silicate'having a 3.25 :1 ratio through a bed of an ion exchange resin in the hydrogen form The sol contained 2.2 per cent SiOaby weight, .had an absolute turbidity of less than *.0226101117 at 1 per cent SiOz with 547 millimicron wavelength light, and a relative viscosity at 5per cent SiO: o-f more'than 1.29.-

A portionof this sol was placed in 'anevaporatorirand was heated to boilingover a period of 20 minutes at atmospheric pressure. The evaporator was equipped with a constant level feed device?- which maintained the volume of liquid in the 'evaporator' at a constant value. After 20 minutesiof-heatingevaporation ofwater was started; andito the heat-treated heel in the evaporatoriresh silica sollwas added as required to keep the volume constant. Evaporation and feedingiwas continuedluntil the product in the evaporator contained 30.23 per cent SiOz by weight. The evaporation time was about 6 hours. The product obtained was stable against gelling for:.:6t months at roomtemperature and 54 days at"9.5 C." The silicazalkali ratio was 7851. The relative viscosity of the product, at 10 per cent S102, was in the range from 1.'15to 1.55 and the molecular weight of the silica particles in the product; as determined by light scattering, was more than one-half. million.

Example 5 A-silica sol was" prepared by the ion exchange procedure" described in Example 4 having the same-characteristics except that the silica'content was 2 per cent SiOz and the ratio was*70.211. Inarr atmospheric evaporator this solution was-heated and boileddown until the S102 content was 30 per cent by weight. The heel thus obtained had a molecular weight of less than 2"-mill'ion as determined by light scattering and a relative'viscosity of from 1.15 to 1.60 at 10 per cent S102.- The addition of fresh 2 per cent Si02 sol was then started and a continuous withdrawal of product was made, the rate of feeding fresh sol and of withdrawing product being so relatedithat the'volume in the evaporator remained'substantially constant and the-loss of waterv by evaporation was suflicient to keep-the concentration of "silica in the evaporator at about 30 pencent; This manner of operationwas continned-for a'total of 11 hours.

The product obtained'had an average silica content of 28.1 per cent by'weight, a molecular weight, by light scattering;v of 1.9 million, an SiOzrNezO weight'ratio of 72: 1 and a relative viscosityyat lOper cent SiOz, in the range from 1.1'5"to--1.55. The product was'stable against gell-ing at 95 C. for more than 5 months and was stable indefinitelyat room temperature.

We'claim':

1. A process comprismgpbuildingzup thesize of particles: in an. aqueous: silica 2 sol from less than 10 millimicrons toalarger size, themaxi, mum size being millimicrons, by formingj'a heel by heating at from 60"-C. to about the boiling temperature at atmospheric pressure-an aqueous sol of silica particles of, less than 10 millimicrons diameter,- the heel, after'heating, having a relative viscosity of from 1.15 to 1.60" at 10% SiO2, addingv to said heel a'silica sol containing "particles of less than lo'millimicrons diameter; and continuingthe-addition and heating until at least 5 times as'much silica has been addedto the heel as was originally present.

25A process comprising building up the size of -particles in an aqueous silicasol from lessztha'n l0 millimicrons to 1 a: larger size, the maximum SiZ billQ'lBU millimicrons, by forming a heel-by heating at from' 60 C'. to-about the boiling-temperature at atmospheic pressure an aqueous sol of silica particles of less than 10-mi1limicrons diameter, the heel, after heating, having-arela tive-'vis'cosityof from 1.15 to 1.60 at'10% SiO'z', adding to saidheel a silica sol containingrpar ticles' ofless-than 10 millimicrons diameter, and continuing the addition and heatinguntil the silica particles in the sol have grown to at least 15 millimicrons diameter.

3. A-process comprising building up the size of particles in an aqueous silica sol fromless than li) millimicrons to a larger size, the maximum-size being 130 millimicrons, by forming a heel by heating at from 60 C. to about the boiling temperature at atmospheric pressure an aqueous sol of silica particles of less'than' 10 milli-' microns diameter, the heeL'after heating, hav ing a relative viscosity of from 1.15 to 1.60'at 10% SiO'a'adding to said heel a silica sol containing particlesof less than 10 millimicrons diameter; while maintaining the pH in the range'from'"9 to 11; and continuing the addition and heating until thesilica particles in the sol have grown to at least-l5 millimicrons diameter.

4. A' proces's' comprising building up the size ofparti'rilesin anaqueous silica sol from 'less than-10 rnil1imicrons to a larger size, the maxi mum size being 130'mi1lim'icrons, by 'forming a heel'by' heating at from 60 C. to about theboil ing temperature at atmospheric pressure an aqueous sol of silica particles of less than" 10 millimicrons diameter, said heel having a silicazalkali ratio of from 60:1 to 130:1, a particle molecular weight of less than fifty million as-de-' t'ermined by light scattering, and a relative viscosity of from 1.15 to 1.60 at 10 per cent SiOz; adding to said heel an aqueous silica sol containing particles of less than 10 millimicrons diameter, having a silicatalkali ratio of from 60:1 to 130:1, an absolute turbidity of less than 0.0226 cmz- 'at 1 percent SiOz with 547 millimicron wavelength "light, and a relative viscosity, at5 percent 3102, of'more" than 1.29, heating-the combined sols to from 60 C. to about the -boil'-' inglxtemperature at atmospheric pressure, and continuing the addition and heating until the weight'of S102 added is at least five times that originallyv present.

5. A: process comprising; building up the size of-particles in an aqueous silica sol" from less than 10 millimicron'stoa larger size, the maxi mum size-being 130' millimicrons, by "forming =2 a heel by heating at from" 60 C- to about the'boilingtemperature at' atmospheric pressure an aqueous sol of silica-particlesof less than 10 millimicrons diameter, said heel having-a silica zaalkali ratiozofdrom' 60; 1: to 13021, a particle determined by light scattering, and a relative viscosity of from 1.15 to 1.60 at 10 per cent S102, adding to said heel an aqueous silica sol containing particles of less than 10 millimicrons diameter, having a silica:alkali ratio of from 60:1 to 130:1, an absolute turbidity of less than 0.0226 cm.- at 1 per cent S102 with 457 millimicron Wavelength light, and a relative viscosity, at 5 per cent SiOz, of more than 1.29, heating the body of combined sols tofrom 60. C. to about the boiling temperature at atmospheric pressure and evaporating water therefrom, and continu ing the addition, heating and evaporation until the weight of SiOz added is at least five times that originally present, the rate of evaporation being so related to the rate of addition of fresh sol as to maintain the volume of the combined sol body substantially constant.

6. A process comprising building up the size of particles in an aqueous silica sol from less than millimicrons to a larger size, the maximum size being 130 millimicrons, by forming a heel by heating at from 60 C. to about the boiling temperature at atmospheric pressure an aqueous sol of silica particles of less than 10 millimicrons diameter, saidheel having a silica:alkali ratio of from 60:1 to 130:1, a particle molecular weight of less than fifty million as determined by light scattering, and a relative viscosity of from 1.15 to 1.60 at 10 per cent SiO2, adding to said heel an aqueous silica sol containing particles of less than 10 millimicrons diametenhaving a silicazalkali ratio of from 60:1 to'130: 1, an absolute turbidity of less than 0.0226 cm? at 1 per cent $102 with 547 millimicron wavelength, light, and a relative viscosity, at 5 per cent S102, of more than 1.29, heating the combined sols to from 60 C. to about the boiling temperature at atmospheric pressure" until the relative viscosity is from 1.15 to 1.55 at 10 per cent S102, withdrawing a portion of the sol body so produced, adding to the remaining sol body a silica sol having a ratio, absolute turbidity, and relative viscosity within the ranges specified for the sol originally added, heating the combined sols from 60 C. to about the boiling tem-' perature at atmospheric pressure, continuing the heating, and continuing the addition at such a rate that'the relative viscosity of the combined sols is maintained in the range from 1.15 to 1.55 at 10 per cent 8102 until the total weight of SiOz added is at least five times that originally present.

7. A process comprising building up the size of particles in an aqueous silica sol from less than 10 millimicrons to a larger size, the maximum size being 130 millimicrons, by forming a heel by heating at from 60 C. to about the boiling temperature at atmospheric pressure an aqueous sol of silica particles of less than 10 millimicrons diameter, said heel having a silicatalkali ratio of from 60:1 to 130:1, a particle molecular weight of less than fifty million as determined by light scattering, and a relative viscosity of from 1.15 to 1.60 at 10 per cent SiO2, adding to said heel an aqueous silica sol containing particles of less than 10 millimicrons diameter, having a silicazalkali ratio of from 60:1 to 130:1, an absolute turbidity of less than 0.0226

cm. at 1 per cent $102 with 547 millimicron wavelength light, and a relative viscosity, at 5 per cent SiOz, of more than 1.29, heating the the relative viscosity is from 1.15 to 1.55 a.t 10 per cent S102, withdrawing a portion of the sol body so produced, adding to the remaining sol body a silica sol having a ratio, absolute turbidity, and relative viscosity within the ranges specified for the sol originally added, heating the combined sols to from 60 C. to about the boiling temperature at atmospheric pressure, continuing the heating, and continuing the addition at such a rate that the relative viscosity of the combined sols is maintained in the range from 1.15 to 1.55 at 10 per cent 'SiQ2 until silica particles in finely divided form precipitate.

8. In a process for modifying a silica sol by building up the size of particles therein from less than 10 millimicrons to a larger size, the maximum size being 130 millimicrons, the steps'comprising adding an aqueous silica sol prepared by removing metal ions from an aqueous metal sil-' icate solution with an acid-treated ion-exchange material and having a silicazalkali ratio of from 60:1 to 130:1, anabsolute turbidity of less'than 0.0226 cm.- at 1 per cent SiOz' with 547 milli micron wavelength light, and a relative viscosity, at 5 per cent 85.02, of more than 1.29, to

, another aqueous silica sol similarly prepared and then heated to from 60 C. to about the boiling temperature at atmospheric pressure, the sol after heating having a silicazalkali ratio of from 60:1 to :1, a molecular weight of less than fifty million as determined by light scattering, and a relative viscosity of from 1.15 to 1.60 at 10 per cent SiOz, and heating the combined sols at from 60 C. to about the boiling temperature at atmospheric pressure, the addition being con-' tinued at such a rate that the relative viscosity of the combined sols is maintained in the range from 1.15 to 1.55 at 10'per.cent SiOz and until the weight of SiO2 added is at least five times that originally present.

9. In a process for modifying a silica $01 by building up the size of particles therein from less than 10 millimicrons to a larger size, the maximum size being 130 millimicrons, the steps comprising adding an aqueous silica sol having a silica:alkali ratio of from 60:1 to 130:1, an absolute turbidity of less than 0.0226 cm. at 1 per cent SiOz with 547 millimicron wavelength light, and a relative viscosity, at 5 per cent S102, of more than 1.29, to another aqueous silica sol having a silicazalkali ratio of from 60:1 to 130:1, a molecular weight of less than fifty million as determined by light scattering, and a relative drawing a portion of the sol body as produced,-

continuously adding to the remaining sol body a silica sol having a ratio, absolute turbidity, and relative viscosity within the ranges specified for the sol originally added, heating the body of combined sols at from 60 C. to about the boiling temperature at atmospheric pressure and evaporating water therefrom," the addition being continued at such a rate that the relative viscosity of the combined sols is maintained in the range from 1.15 to 1.55 at 10 per cent SiOa and until the total weight of Si02 added is at least five times that originally present, and the rates of evaporation and withdrawal being so related to the rate of addition of fresh 501 as to maintain the volume of the combined sol body substantially constant. 7

10. In a process for modifying a silica sol by building up the size of particles therein from less than 10 millimicrons to a larger size, the maximum size being 130 millimicrons, the steps comprising adding an aqueous silica sol prepared by removing metal ions from an aqueous metal silicate solution with an acid-treated ion-exchange material and having a silicazalkali ratio of from60z1 to 130:1, an absolute turbidity of less than 0.0226 cmr at 1 per cent SiOa with 547 millimicron wavelength light, and a relative viscosity, at 5 per cent S102, of more than 1.29, to another aqueous silica sol similarly prepared and then heated to from 60 C. to about the boiling temperature at atmospheric pressure, the

sol after heating having a silicazalkali ratio of from 60:1 to 130:1, a molecular weight of less than fifty million as determined by light scattering, and a relative viscosity of from 1.15 to 1.60 at 10 per cent SiOz, and heating the body of combined sols at from 60 C. to about the boiling temperature at atmospheric pressure and evaporating water therefrom, the addition being continued at such a rate that the relative viscosity of the combined sols is maintained in the range from 1.15 to 1.55 at 10 per cent SiOa, and until the weight of SiOz added is at least five times that originally present, and the rate of evaporation being so related to the rate of addition of fresh sol as to maintain the volume of the combined sol body substantially constant.

11. In a process for modifying a silica sol by building up the size of particles therein from less than 10 millimicrons to a larger size, the maximum size being 130 millimicrons, the steps comprising boiling an aqueous silica sol, freshly prepared by removing sodium ions from sodium silicate solution with an acid-treated ion-exchange material and having an SiOzINazO ratio of from 70:1 to 100:1 and an Si02 content from 2 to 6 per cent, until its relative viscosity is from 1.15 to 1.60 at 10 per cent SiO'z, continuously adding more of the original silica sol while boiling the body of combined sols to evaporate water therefrom, the'rate of sol addition being equal to the rate of evaporation and being such as to maintain the relative viscosity of the combined sol body in the range from 1.15 to 1.55 and the addition and evaporation being continued until the sol contains from to 35 per cent SiOz.

12. In a process for modifying a silica sol by building up the size of particles therein from less than 10 millimicrons to a larger size, the maximum size being 130 millimicrons, the steps comprising boiling an aqueous silica sol, freshly prepared by removing sodium ions from sodium silicate solution with an acid-treated ion-exchange material and having an SiO2:Na2O- ratio of about 85:1 and an SiQz content of about 2.5 per cent, until its relative viscosity is from 1.15 to 1.60 at 10 per cent SiOz, continuously adding more of the original silica sol while boiling the body of combined sols toevaporate water therefrom, the rate of sol addition being equal to the rate of evaporation and being such as to maintain the relative viscosity of the combined sol body in the range from 1.15 to 1.55 and the addition and evaporation being continued until the sol contains about per cent SiO2.

13.- An aqueous, stable silica sol characterized by having a silica:alkali ratio of from 60:1 to 130:1, by containing discrete silica particles 18 having a molecular weight, as determined by light scattering, of more than one-half million, by having a relative viscosity, at 10 per cent SiOz, from 1.15 to 1.55, and by containing from 20 to 35 per cent by weight of S102.

14. An aqueous, stable silica sol characterized by having a silica:NazO ratio of from :1 to 100:1, by containing discrete silica particles having a molecular weight, as determined by light scattering, of more than one-half million, by having a relative viscosity, at 10 per cent SiO2, from 1.15 to 1.55, and by containing from 20 to 35 per cent by weight of $102.

15. An aqueous, stable silica sol characterized by having a si1ica:Na2O ratio of from 70:1 to 100:1, by containing discrete silica particles having a molecular weight, as determined by light scattering, of from A to million, by having a relative viscosity, at 10 per cent SiOz, from 1.15 to 1.55, and by containing from 20 to 35 per cent by weight of SiOz.

16. A process comprising building up the size of particles in an aqueous silica sol by heating 'to from 60 C. to about the boiling temperature at atmospheric pressure a heel consisting of an aqueous sol of silica particles, the sol, after heating, having a relatively viscosity of from 1.15 to 1.60 at 10% S102, adding to said heel a silica sol containing particles of less than 10 millimicrons diameter, and continuing the addition and heating until at least 5 times as much silica has been added to the heel as was originally present.

17. A process comprising building up the size of particles in an aqueous silica sol by heating to from 60 C. to about the boiling temperature at atmospheric pressure a heel consisting of an aqueous sol of silica particles, the sol, after heating, having a relative viscosity of from 1.15 to 1.60 at 10% SiOz, adding to said heel a silica sol containing particles of less than 10 millimicrons diameter, and continuing the addition and heating until the silica particles in the sol have an average diameter of at least 15 millimicrons.

18. A process comprising building up the size of particles in an aqueous silica sol by heating to from 60 C. to about the boiling temperature at atmospheric pressure a heel consisting of an aqueous sol of silica particles, the sol, after heating, having a relative viscosity of from 1.15 to 1.60 at 10% 8102, adding to said heel a silica sol containing particles of less than 10 millimicrons diameter, while maintaining the pH of the mixture in the range from 9 to 11, and continuing the addition and heating until at least 5 times as much silica has been added to the heel as was originally present.

MAX F. BECHTOLD. OMAR E. SNYDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. A PROCESS COMPRISING BUILDING UP THE SIZE OF PARTICLES IN AN AQUEOUS SILICA SOL FROM LESS THAN 10 MILLIMICRONS TO A LARGER SIZE, THE MAXIMUM SIZE BEING 130 MILLIMICRONS, BY FORMING A HEEL BY HEATING AT FROM 60* C. TO ABOUT THE BOILING TEMPERATURE AT ATMOSPHERIC PRESSURE AN AQUEOUS SOL OF SILICA PARTICLES OF LESS THAN 10 MILLIMICRONS DIAMETER, THE HEEL, AFTER HEATING, HAVING A RELATIVE VISCOSITY OF FROM 1.15 TO 1.60 AT 10% SIO2, ADDING TO SAID HEEL A SILICA SOL CONTAINING PARTICLES OF LESS THAN 10 MILLIMICRONS DIAMETER, AND CONTINUING THE ADDITION AND HEATING UNTIL AT LEAST 5 TIMES AS MUCH SILICA HAS BEEN ADDED TO THE HEEL AS WAS ORIGINALLY PRESENT. 